Acoustic Wave Generation in Nanofluids: Effect of the Kapitza Resistance on the Thermophone to Mechanophone Generation Mechanism Transition

High frequency, short wavelength acoustic wave generationin fluids,by means of immersed nanotransducers, is key to applications rangingfrom the biomedical to the ICT sector, all the way to investigatingfundamental aspects of the mechanics of fluids. In this context, thecommonly adopted generation mechanism is the thermophone, where thenanotransducer serves as a nanoheater for the surrounding fluid andthe acoustic wave is launched by water thermal expansion. Its performance,however, is severely degraded when reaching up to hypersonic frequencies.We analytically investigate the thermoacoustic response in the frequencydomain for the case of a gold nanofilm transducer in contact withwater. We prove that another generation mechanism, the mechanophone,sets in this critical frequency range, widening the acoustic generationbandwidth. In the mechanophone, it is the nanotransducer thermal expansionthat launches pressure waves in water. We find a threshold frequencydiscriminating between these two regimes, which can be tuned by actingon the Kapitza resistance at the solid-liquid interface. Wethen show how the mechanophone mechanism can be activated in the frameof photoacoustic generation with pulsed laser sources. The unveiledphysics bears generality beyond the specific system and explains theacoustic generation mechanisms in nanofluids.